The saframycins, isolated in the late 1970s, are a family of microbial fermentation products with significant anti-proliferative activity and anti-microbial activity against gram-positive bacteria (Arai, T.; Kubo, A. In The Alkaloids; Brossi, A., Ed.; Academic Press: New York, 1983; Vol. 21, Chapter 3; Arai, T.; Takahashi, K. the Journal of Antibiotics 1977, 30, 1015–1018). Several saframycin analogues have been isolated and characterized in recent years (see, e.g., DE 2839668; U.S. Pat. No. 4,248,863; U.S. Pat. No. 4,372,947; U.S. Pat. No. 5,023,184; U.S. Pat. No. 4,837,149; and EP 329606; each of which is incorporated herein by reference). For example, saframycins A-H, R, and S have been isolated from the culture broths of Streptomyces lavendulae, and saframycins Mx1 and Mx2, have been isolated from the culture broths of the myxobacterium, Myxococcus xanthus, each of the saframycins varying in the oxidation state of the ring system and in substitution of the core structure (Saito et al. Chem. Pharm. Bull. 43:777, 1995). Certain saframycins, namely A and C, exhibit extreme cytotoxicity toward cultured cells and toward several experimental tumor cell lines including leukemias L1210 and P388 and Ehrlich carcinoma (Arai, T.; Mikami, Y.; Okamoto, K.; Tokita, H.; Teras, K. In Advances in cancer chemotherapy; University Park Press, Baltimore, 1978, 235–251; Arai, T.; Takahashi, K.; Ishiguro, K.; Mikami, Y. Gann 1980, 71, 790–796; Ishiguro, K.; Sakiyami, Takahashi, K.; Arai, T. Biochemistry 1981, 17, 2545–2550; Arai, T.; Takahashi, K.; Nakahara, S.; Kubo, A. Experientia 1980, 36, 1025–1027; Myers, A. G.; Plowright, A. T. J. Am. Chem. Soc. 2001, 123, 5114; Martinez, E.; Owa, T.; Schreiber, S. L.; Corey, E. J. Proc. Natl. Acad. Sci. USA 1999, 96, 3496–3501).
TABLE 1IC50s in Tumor Cell LinesA375A549 lungCompoundmelanomacarcinoma(−)-Saframycin A5.3nM133nMQAD1.2nM4.4nMEcteinascidin 7430.15nM1.0nMSaframycin A has been shown to block RNA synthesis in cultured cells, and it has been suggested that saframycins A and C exhibit this potency because of their ability to bind covalently to DNA (for a discussion of the biological activity of the saframycins see, for example, Lown et al. Biochemistry, 1982, 21, 419; Ishiguro et al. Biochemistry, 1978, 17, 2545; Rao et al. Chem. Res. Toxicol., 1990, 3, 262, 1990; Ishiguro et al. J. Biol. Chem., 1981, 256, 2162).
Saframycin A (SafA), the most potent member of this series, exhibits wide spectrum anti-cancer activity against Ehrlich ascites tumor, B16 melanoma, and murine leukemias P388 and L1210 (Arai, T.; Mikami, Y.; Okamoto, K.; Tokita, H.; Teras, K. In Advances in cancer chemotherapy; University Park Press, Baltimore, 1978; pp 235–251; Arai, T.; Takahashi, K.; Ishiguro, K.; Mikami, Y. Gann 1980, 71, 790–796; Ishiguro, K.; Sakiyami, Takahashi, K.; Arai, T. Biochemistry 1981, 17, 2545–2550; Arai, T.; Takahashi, K.; Nakahara, S.; Kubo, A. Experientia 1980, 36, 1025–1027). A structurally related class of non-quinoid natural products, the ecteinascidins, was found to possess an even more potent antiproliferative activity (Rinehart, K. L.; Holt, T. G.; Fregeau, N. L.; Keifer, P. A.; Wilson, G. R.; Perun, T. J.; Sakai, R.; Thompson, A. G.; Stroh, J. G.; Shield, L. S.; Seigler, D. S.; Li, L. H.; Martin, D. G.; Grimmelikhuijzen, C. J. P.; Gäde, G. Journal of Natural Products 1990, 53, 771–792; Sakai, R.; Rinehart, K.; Guan, Y.; Wang, A. H. J. Proc. Natl. Acad. Sci. USA 1992, 89, 11456–11460). One member of the class, ecteinascidin 743 (Et-743), has advanced to Phase III clinical trials, and it was found to be particularly active against soft tissue sarcomas without obvious toxicity (Jimeno, J. M.; Faircloth, G.; Cameron, L.; Meely, K.; Vega, E. Gomez, A.; Sousa-Faro, J. M. F.; Drugs Future 1996, 21, 1155–1165; Bowman, A.; Twelves, C.; Hoekman, K.; Simpson, A.; Smyth, J.; Vermorken, J.; Hoppener, F.; Beijnen, J.; Vega, E.; Jimeno, J. Hanauske, A. R. Ann. Oncol. 1998, 9 (Suppl. 2), 119; Taamma, A.; Misset, J. L.; Riofrio, M. Guzman, C.; Brain, E.; LopezLazaro, L.; Rosing, H.; Jimeno, J. M.; Cvitkovi, E. J. Clin. Oncol. 2001, 19, 1256–1265). By employing an efficient synthetic route to SafA, a number of saframycin structural analogues have been prepared (see U.S. Ser. No. 10/011,466, filed Nov. 5, 2001; U.S. Ser. No. 60/245,888, filed Nov. 3, 2000; each of which is incorporated herein by reference). The most active analogue, the quinaldic acid analogue of SafA (QAD), possesses nearly 30-fold greater activity than SafA in a lung carcinoma cell line and 4-fold greater activity in a melanoma cell line. QAD has been reported to show 100-fold greater potency in three human sarcoma cell lines as compared to Et-743 (Myers, A. G.; Kung, D. W., J. Am. Chem. Soc. 1999, 121, 10828–10829; Myers, A. G.; Plowright, A. T. J. Am. Chem. Soc. 2001, 123, 5114–5115).

Due to the potent antiproliferative activity of the ecteiniscidins, saframycins, and related analogues, elucidating their biological mode of action has been an area of active research ever since their discovery. Elucidating the biological target of the saframycins may lead to the development of assays which can be used to identify better pharmaceutical agents with the same mode of action as the saframycins. Also, since many other anti-proliferative agents are known to bind DNA, an activity which is believed to be central to their biological activities, the assays developed for use with saframycins may be applicable to other DNA-binding agents.